1. Field of the Invention
An object of the present invention is a method to localize or determine the position of a mobile telephone in a network of base transceiver stations. An object of the invention is to provide new possibilities for the use of mobile telephones and to improve radiocommunications systems.
2. Description of the Prior Art
In the field of mobile telephony, it is becoming increasingly necessary to localize or determine the position of mobile telephones during conversation and, possibly, even all the time. In practice, in the U.S., the FCC (the Federal Communications Commission) stipulates that mobile telephones should be located with a precision of less than 125 meters, for 67% of the time. In other words, position-finding is becoming a necessity for mobile radiotelephones of future generations.
Briefly, determining the position of mobile telephones enables a mobile telephone to be located, during use, when it has been stolen. This may provide indications on how to find the thief. Furthermore, in the event of an emergency, the sole fact that a user communicates his location may enable action by rescue workers. More generally, geographical localization makes it possible to supply users with messages, possibly advertisements, related to the place in which they are located.
The methods of localization that can be envisaged include the installation, in each mobile telephone, of a GPS (Global Positioning System) detector by which the mobile telephone would pick up transmissions from satellites in order to know its position. Although this technique is quite developed, it is not possible, for reasons of cost, to consider installing it at present in mobile telephones. For, it necessitates the use of specific equipment.
Other solutions can be considered and are described in Chris Drane and Chris Rizos, "Positioning Systems in Intelligent Transportation Systems," Artech House Books, Boston & London, Jan. 31, 1998. This document considers the carrying out of localizing operations by angle of arrival. This system however requires the making of numerous overhead lines to discriminate between radioelectric signals received on several lobes and designate a direction of arrival of the transmissions received. On the basis of at least three directions (on the basis of at least three base transceiver stations), it is possible to locate the position of the sending mobile telephone. This approach has the drawback wherein numerous overhead lines requiring lots of space have to be installed for each base transceiver station, on the frontage of the houses. This cannot always be done, especially for decorative reasons. Furthermore, in this case, it may happen that the mobile cannot be localized. It must transmit so that the base transceiver stations can localize it.
A second approach envisaged is a time measurement. Using a measurement of a date of arrival of radioelectric signals at a base transceiver station or a mobile telephone, it is possible in theory to determine the distance of a mobile telephone from a base transceiver station. By combining information on the distance of the mobile telephone from several base transceiver stations, it is possible to determine the location of this mobile telephone.
In the prior art, a measurement of arrival time is already performed by a base transceiver station, in existing installations. This measurement is aimed at ensuring that transmissions coming from a mobile telephone reach this base transceiver station so that they are synchronized with the reception frame of this base transceiver station. Thus, especially within the context of GSM type mobile telephony, in 4.6 millisecond frame, there are eight 577-microsecond temporal windows. Given that a geographical cell governed by a base transceiver station can have a size with a diameter of 30 kilometers, the maximum distance of propagation envisaged for the propagation of a wave is about 30 kilometers. For propagation at the speed of light, this leads to phase delays of up to 300 microseconds. This is a considerable amount of time in relation to the duration of the temporal windows. In order to obtain, at the position of a base transceiver station, a presence of significant bits during 5-microsecond microwindows (corresponding to a channel passband of 200 Khz), it has been planned to measure arrival times with 60 delay levels, constituting in practice 64 levels.
For a mobile telephone very close to the base transceiver station, the temporal delay will be zero.
For a distant mobile telephone, the time lag and, therefore, a time advance TA, should corresponding to 300 microseconds. To inform the mobile of the duration of time advance TA to be implemented, a base transceiver station sends it a TA message encoded on six bits (for 64 levels) enabling this lead to be determined.
In practice, this lead is synchronous with a period of propagation, and it is also synonymous with a distance of the mobile from the base transceiver station in 64 concentric rings around it: namely rings with widths of about 1100 meters. Although the precision thus obtained is not insignificant, it is not sufficient to correspond to the requirements of the standard. Ultimately, the use of the TA information representing this time advance cannot be exploited to locate the mobile telephone with sufficient precision. Modifying this technique to increase the precision of the TA information would furthermore have the drawback of requiring the technological modification of all the base transceiver stations and all the mobile telephones: this is unrealistic.
It is an object of the invention to overcome these drawbacks, namely to enable effective localizing through the use of existing equipment without requiring any technological modification, and with the required precision under present conditions. The approach of the invention consists, in its principle, in sending a message from a base transceiver station to a mobile telephone. This message has a piece of information on an absolute time pertaining to this base transceiver station. The mobile telephone which receives the message knows, with respect to its own clock, the time at which it receives this message. It also knows how it should furthermore try and reset its internal clock so that it corresponds to the absolute clock whose characteristics have just been sent to it. This sending of absolute time is however affected by an error due to the duration of propagation between the base transceiver station and the mobile telephone.
In the invention, the mobile telephone then sends a response message to the base transceiver station that has sent it the absolute time. This base transceiver station receives this response message. On the basis of the two periods of propagation in the outgoing direction and the return direction between itself and the mobile telephone, the base transceiver station computes a corresponding correction of absolute time. In a second sending operation, the base transceiver station sends information on this correction to the mobile telephone. In receiving this correct information, the mobile telephone, this time permanently, sets its internal clock with respect to the absolute time of the base transceiver station.